In heat exchangers of this type, the primary fluid generally circulates within the tubes of the bundle, whereas the secondary fluid circulates outside the tubes. More specifically, the secondary fluid is injected into the annular space formed between the outer envelope and the bundle envelope, generally into the upper part of the space, so as to drop into the aforementioned annular space and then rise again within the bundle envelope, benefitting from the heat supply provided by the primary fluid circulating in the tubes.
A known solution for injecting secondary fluid into the upper part of the annular space formed between the outer envelope and the bundle envelope is illustrated by FR-A-2 477 265. The secondary fluid penetrates the steam generator by a supply pipe, which is tightly connected to a toroidal or semi-toroidal, sealed supply collector arranged circumferentially in the top of the annular space. On its upper generatrix, the toroidal or semi-toroidal collector has holes to which are connected inverted J-shaped tubes, ensuring the injection of the secondary fluid into the annular space.
This method for injecting the secondary fluid in heat exchangers suffers from a certain number of disadvantages.
Firstly, the special shapes of the injection tubes for the secondary fluid and the upper part of the annular space in which the tubes are housed, as well as the high speed of the secondary fluid jets passing out of these tubes, lead to eddies in the upper part of the annular space. These eddies are prejudicial, because they reduce the thermal efficiency of the heat exchange, particularly when it is a preheating exchanger.
To obviate this disadvantage, FR-A-2 644 926 proposes various solutions, one of which consists of partly sealing the annular space by a non-return plate and downwardly extending the inverted J-shaped tubes by extensions passing through the non-return plate and which are fixed to the latter. However, the structure of the system making it possible to inject the secondary fluid into the heat exchanger then becomes particularly complex and costly.
Moreover, the integration of this structure into the production cycle for the heat exchangers causes problems which are difficult to solve, bearing in mind the manufacturing tolerances which lead to axial misalignments, which are variable for each inverted J-shaped tube, between the lower end of the tubes (not yet provided with their extensions) and the corresponding perforations formed in the non-return plate, in order to permit the passage of the extensions. These problems have the consequence of unduly extending the production cycle for the heat exchangers.
The fixing of the extensions to the non-return plate and to the ends of the inverted J-shaped tubes also leads to a risk of the extensions fracturing under the effect of mechanical stresses produced by differential expansions occurring between operating periods and stoppage periods of the exchanger.
In U.S. Pat. No. 3 913 531 and U.S. Pat. No. 3 906 905, the secondary water supply of a steam generator is also ensured by means of a toroidal collector placed in the top of the annular space formed between the outer envelope and the bundle envelope. On its lower generatrix, the toroidal collector has holes, which issue downwards and directly into the annular space.
Although this arrangement is particularly simple, it also leads to the formation of eddies in the top of the annular space. Moreover, stopping the secondary water supply brings about a complete emptying of the collector, so that water-hammer may occur on resuming the supply.